Some cell systems, such as the lithium/oxyhalide cell system or the lithium/MnO.sub.2 cell system, preferably utilize a hermetically sealed housing. Specifically, cell systems employing lithium, high vapor pressure organic solvents and inorganic cathode/solvent materials are required to be hermetically sealed in order to prevent the exposure of the highly reactive and corrosive cell materials to the outside environment and to provide superior shelf life. Consequently, venting-type hermetic seals have been employed in the battery art and in applicant's copending application Ser. No. 248,215 a hermetic seal is disclosed for electrochemical cells, such as nonaqueous cells, comprising a flexible metallic cover sealed to the open end of a container housing the cell components and said flexible cover having a radially inwardly disposed flange defining an opening through which a conductive terminal of the cell passes. The terminal is hermetically sealed to the cover by a layer of glass or ceramic material bonded to both the flange and the terminal. The flexible cover is formed such that a predetermined pressure buildup within the cell will cause the cover to flex sufficiently so as to fracture the glass or ceramic material or the bond between the cover and the glass or ceramic, or between the terminal and the glass or ceramic, thereby forming a vent passage from the cell. The disclosure made in this application is incorporated herein by reference.
U.S. Pat. No. 4,115,629 discloses a cell closure which includes a flexible metal member which has an aperture of predetermined area over which a non-polarized metallic disc of a size larger than the aperture is symmetrically positioned and a glass or ceramic layer is then hermetically bonded to the closure and to the disc. The wall structure of the bond is formed so that a predetermined gas buildup within the cell will cause the wall to rupture thereby venting the gas from the cell.
U.S. Pat. No. 4,127,702 discloses a self-venting battery wherein an insulating material such as glass or ceramic is provided between and bonded to the terminals for electrically insulating them from one another and wherein the insulator is such that increasing pressure within the battery will cause the insulator to crack a sufficient amount to vent the pressure in the battery before causing the bond between the insulator and terminals to fail.
Although venting-type hermetic seals employing glass or ceramic materials have been used with some success, they are sometimes subjected to degradation or corrosion when exposed to certain environments. In U.S. Pat. No. 4,168,351 a means is disclosed for preventing deterioration of a glass-to-metal seal in a lithium cell which consists of protecting the exposed interior of the glass with a metal oxide or a nonconductive, nonreactive polymeric material such as a fluorocarbon or a polyolefin stable in the presence of lithium and other cell components.
It is an object of the present invention to provide a hermetic seal for electrochemical cells that can withstand high humidity environments without deteriorating or succumbing to corrosion, the corrosion products in some cases being electrically conductive and possibly resulting in wasteful discharge of the cell.
It is another object of the present invention to provide a hermetic seal that is coated on the exterior with an adherent, nonconductive, water-resistant, protective material so as to prevent corrosion in the seal which could result in electrolyte leakage.
It is another object of the present invention to provide an effective corrosion-free hermetic seal for lithium cells.
It is another object of the present invention to provide a method for effectively preventing hermetic seal corrosion in electrochemical cells such as lithium electrochemical cells.